1. Field of the Invention
This invention relates to an elongated magnetic material element adapted to be wound around a conductor of an overhead transmission line to prevent the freezing or icing thereof.
2. Prior Art
A conductor of an overhead transmission line is subjected to icing and the deposition of snow in cold districts during the winter. The snow or the ice on the conductor grows upon lapse of time to increase the weight of the conductor and a wind pressure to which the conductor is subjected, thereby excessively increasing a tension of the conductor, and a sag of the conductor between each adjacent pylons is unduly increased. As a result, the conductor tends to be broken off, and the steel pylons supporting the conductor tend to fall. Further, there is a risk that lumps of snow or ice drop from the conductor and hit a passer-by passing beneath the transmission line. Even if the transmission line is laid over agricultural lands, such fallen lumps of snow or ice may give rise to damage to the crops and farm facilities.
In order to prevent the conductor from being subjected to the deposition of snow and the icing, it has been proposed to temporarily pass a large amount of alternating current through the conductor to generate joule heat by which the snow or ice on the conductor is melted. However, this method can not be carried out at all times because of the limitations on the operation of the transmission line.
Another method of overcoming the above-mentioned difficulty is to mount rings on the conductor in spaced relation to cause the snow on the conductor to drop therefrom. However, the rings often fail to cause the snow or the ice to drop satisfactorily. Further, lumps of the snow or ice caused to drop by the rings may injure a passer-by or cause damage to the crops and the farm facilities.
It has also been proposed to mount a magnetic material element on the transmission line conductor so that the snow or the ice on the conductor is melted by the heat due to hysteresis loss and eddy current loss generated by the magnetic field developing in the magnetic material element due to the flow of alternating current through the conductor. The magnetic material element includes a wire, a tape and a rod all of which are adapted to be spirally wound around the conductor, and a sleeve adapated to be fitted on the conductor. Such magnetic material element should be as lightweight as possible to prevent the transmission line from becoming unduly heavy. Also, since the heat generated by the magnetic material element at temperatures causing no icing or snow deposition contributes to the loss of the transmission power, the magnetic material element should preferably be made of a low Curie point material of which magnetic properties are lowered at high temperatures to generate less heat. Generally, a low Curie point material tends to be less magnetic even at low temperatures than a high Curie point material. Therefore, the melting of snow or ice can not satisfactorily be achieved only by the heat due to the hysteresis loss, and the heat due to the eddy current loss must also be used together to achieve a desired melting of the snow or ice.
Usually, the magnetic material element comprises a magnetic material and a conductive metal sheathing covering it.
In the case of a magnetic material having a high Curie point of not less than 300.degree. C., the heat due to the hysteresis loss is greater than the heat due to the eddy current loss. Therefore, the melting effect is not so affected by the thickness of the conductive metal sheathing covering the magnetic material.
In the case of a magnetic material having a low Curie point of not larger than 200.degree. C., the heat due to the eddy current loss is greater than the heat due to the hysteresis loss. Therefore, it is necessary to properly determine the thickness of the conductive metal sheathing in order to achieve a desired melting of the snow or ice.
The magnetic material element has been made of alloys having a Curie point of 0.degree. to 100.degree. C., such as an alloy containing iron, nickel, chrominum and silicon and having a Curie point of around room temperature. However, magnetic properties of such alloys are liable to be affected by heat treatment conditions and other processing conditions. In addition, such alloys have a poor reproducibility. For example, the magnetic material element in the form of a wire is manufactured by drawing. Magnetic properties of the thus drawn wire are lowered due to the residual strain of the wire irrespective of the reduction rate of the drawing operation. If it is intended to use such a wire for the purpose of melting the snow or ice on the conductor, a large amount of wire must be wound around the conductor to achieve the desired melting.